Fuel cell system with partial external reforming and direct internal reforming

ABSTRACT

A fuel cell system includes a plurality of solid oxide fuel cells arranged in a fuel cell stack, an integrated heat exchanger/reformer operable to partially reform an anode feed prior to entry into the fuel cell stack, an anode tailgas oxidizer, and an offgas flow path extending away from an anode side of the fuel cell stack and having a first branch to selectively combine offgas from the anode side of the fuel cell stack with fuel from a fuel source to comprise the anode feed to the fuel cell stack and a second branch to supply offgas from the anode side of the fuel cell stack to the anode tailgas oxidizer. The integrated heat exchanger/reformer transfers heat from the oxidized offgas from the anode tailgas oxidizer to the anode feed before the anode feed enters the anode side of the fuel cell stack. The offgas from the anode tailgas oxidizer provides the sole heat source for the anode feed traveling through the integrated heat exchanger/reformer.

RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No.60/923,885 filed on Apr. 17, 2007, the contents of which are includedherein by reference.

FIELD OF THE INVENTION

The present invention relates to fuel cells, and in particular, to solidoxide fuel cells.

SUMMARY OF THE INVENTION

The fuel cell system of the present invention can have both partialexternal reforming and direct internal reforming means and the amount ofdirect internal reforming can be adjusted through a simplified controlmeans and simplified flow routing.

The present invention can also or alternatively provide a fuel cellsystem capable of operation at high thermal efficiency levels.

The present invention can also or alternatively provide a process forrapidly starting a fuel cell system having both integrated partialexternal reforming and direct internal reforming means and for stablyoperating such a system at low power output levels.

In some embodiments, the present invention provides a fuel cell systemcomprising a plurality of solid oxide fuel cells arranged in one or morefuel cell stacks, a heat exchanger/reformer to preheat and partiallyreform the anode feed to the fuel cells, and an anode tailgas oxidizer(ATO) to provide a hot exhaust gas as the sole heat source for the heatexchanger/reformer, wherein the cooled anode offgas is split into afirst portion to be oxidized in the ATO and a second portion to berecycled back to the fuel cell anodes, and wherein that portion of thefuel not reformed within the heat exchanger/reformer is directinternally reformed at the fuel cell anodes. The relative amount ofdirect internal reforming is controlled through varying the fuelutilization. Therefore, at high levels of fuel utilization, thepercentage of fuel that is internally reformed is high, and at lowlevels of fuel utilization, the percentage of fuel that is internallyreformed is low.

The invention also provides a fuel cell system including a plurality ofsolid oxide fuel cells arranged in a fuel cell stack, an integrated heatexchanger/reformer operable to partially reform an anode feed prior toentry into the fuel cell stack, an anode tailgas oxidizer, and an offgasflow path extending away from an anode side of the fuel cell stack andhaving a first branch to selectively combine offgas from the anode sideof the fuel cell stack with fuel from a fuel source to comprise theanode feed to the fuel cell stack and a second branch to supply offgasfrom the anode side of the fuel cell stack to the anode tailgasoxidizer. The integrated heat exchanger/reformer transfers heat from theoxidized offgas from the anode tailgas oxidizer to the anode feed beforethe anode feed enters the anode side of the fuel cell stack. The offgasfrom the anode tailgas oxidizer provides the sole heat source for theanode feed traveling through the integrated heat exchanger/reformer.

In other embodiments, the invention provides a method of operating afuel cell system. The method includes the acts of: providing a fuel cellstack including a plurality of solid oxide fuel cells, at leastpartially reforming an anode feed in an integrated heatexchanger/reformer, supplying the at least partially reformed anode feedto an anode side of the fuel cell stack, removing offgas from the anodeside of the fuel cell stack along an offgas flow path, combining a firstportion of offgas from the anode side of the fuel cell stack with theanode feed to recycle the offgas back to the fuel cell stack, supplyinga second portion of offgas from the anode side of the fuel cell stack toan anode tailgas oxidizer, transferring heat from the oxidized offgasfrom the anode tailgas oxidizer to the anode feed in the integrated heatexchanger/reformer, and adjusting relative volumes of the anode feedreformed by the integrated heat exchanger/reformer and directlyinternally at the anode side of the stack by adjusting the heat contentof the oxidized offgas.

In other embodiments, the invention provides a fuel cell systemincluding a fuel cell stack having an anode side, an integratedreformer/heat exchanger, a first offgas flow path extending through afirst pass of the integrated reformer/heat exchanger, and a secondoffgas flow path extending through a second pass of the integratedreformer/heat exchanger and in heat exchange relation with offgasflowing through the first offgas flow path. The second offgas flow pathis operatively connected to the inlet of the anode side of the fuel cellstack downstream of the integrated reformer/heat exchanger to deliver ananode feed comprising recycled anode offgas to the anode side of thefuel cell stack. An upstream portion of the first offgas flow path isfluidly connected to an upstream portion of the second offgas flow path.

In other embodiments, the invention provides a method of operating afuel cell system. The method includes the acts of operating the fuelcell according to a first mode of operation for starting the fuel cellsystem including directing offgas from an anode side of a fuel cellstack through compressor and a startup burner for preheating a cathodefeed with the compressed and heated anode offgas, operating the fuelcell system according to a second mode of operation including directingoffgas from the anode side of the fuel cell stack through an oxidizerand an integrated heat exchanger/reformer downstream of the oxidizer,and operating the fuel cell system according to a third mode ofoperation including recycling anode offgas through the anode side of thefuel cell stack.

The above and other features, aspects and advantages of the presentinvention will become apparent from a review of the detailed descriptionand drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a portion of a fuel cellsystem embodying the present invention.

FIG. 2 is a graph illustrating the temperature profiles of an anode feedflow and an exhaust flow in a heat exchanger/external reformer in a fuelcell system embodying the present invention, as calculated in ananalysis of the fuel cell system at one operating condition.

FIG. 3 is a graph illustrating the anode feed flow composition along thelength of the heat exchanger/external reformer from the same systemanalysis as the graph of FIG. 2.

FIG. 4 is a graph of FIG. 2 but at another operating condition.

FIG. 5 is a graph of FIG. 3 but at the operating condition of FIG. 4.

FIG. 6 is a diagrammatic representation of a portion of a fuel cellsystem embodying the present invention similar to FIG. 1 but showingadditional aspects.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

Relevant portions of a solid oxide fuel cell system 1 embodying thepresent invention are shown in FIG. 1. The fuel cell system 1 comprisesa plurality of fuel cells 2, each fuel cell comprising a solidelectrolyte 3 separating an anode 4 and a cathode 5. The fuel cellsystem 1 also comprises a heat exchanger/external reformer 6 and ananode tailgas oxidizer (ATO) 7. It should be understood that the term“external reformer” when used herein is not meant to imply that thedevice is external to or not integrated into the fuel cell system or anysub-part of the system, but is instead used to denote that the source ofheat for the reforming reaction is not heat that is directly rejectedfrom the stack to the reformer via conduction or radiation heattransfer, a mode of operation commonly referred to as “internalreforming”. It will be shown later in the detailed description that thedegree to which the fuel can be reformed within the heatexchanger/external reformer 6 is limited by the enthalpy content of aheating flowstream passing therethrough, and the device 6 will at timesbe referred to as a “Heat exchanger and Enthalpy Limited PartialExternal Reformer” (HELPER).

The embodiment of FIG. 1 is further comprised of a recuperative heatexchanger 9 for the anode flow, a recuperative heat exchanger 8 for thecathode flow, an air preheater/anode offgas cooler 10, an anode offgasrecycle valve 11, and an anode recycle pressurization device 12 (e.g., acompressor, a venturi, etc.). Although these devices are all shown inthe embodiment of FIG. 1, it should be appreciated that not all of thesedevices are required to be present in an embodiment of the invention andno limitation requiring the presence of any or all of these devices inthe invention is intended.

In the embodiment of FIG. 1, an anode feed 22 comprised of a gaseoushydrocarbon fuel flow 21 and a pressurized recycled anode offgas flow 20enters the HELPER 6. Within the HELPER 6, the flow 22 is heated to atemperature suitable for methane steam reforming, and some fraction ofthe hydrocarbon species in the flow 22 are reformed so that the flowexits the HELPER 6 as a reformate flow 15 comprised of methane,molecular hydrogen, water vapor, carbon monoxide and carbon dioxide, andfree of any substantial concentration of hydrocarbons of a higher orderthan methane. The source of heat for the heating and reforming of theflow 22 is an ATO exhaust flow 29 that passes through the HELPER 6 inheat exchange relation with the flow 22, and exits the HELPER 6 as acooled exhaust flow 30.

In the embodiment of FIG. 1, the reformate flow 15 next flows throughthe anode recuperative heat exchanger 9 and is further heated therein toa temperature that is compatible with the fuel cells, exiting as a fullyheated reformate flow 13, which is routed to the fuel cell anodes 4. Atthe fuel cell anodes 4, the remaining hydrocarbons in the fully heatedreformate flow 13 are directly internally reformed, and some fraction ofthe oxidizable fuel species is electrochemically oxidized by the oxygenions transported through the electrolyte 3 from the cathode 5, therebyproducing a flow of electrons from anode to cathode through an externalelectrical circuit connected to the fuel cell system 1. The rate atwhich oxygen ions are transported to the anode 4 as a percentage of therate that would be required to fully oxidize the fuel delivered to theanode 4 by the heated reformate flow 13 is referred to as the anodeutilization of the fuel cells.

As further illustrated in FIG. 1, the flow exits the anode as an anodeoffgas flow 14 and passes through the anode recuperative heat exchanger9, wherein it rejects heat to reformate flow 15 and exits as a partiallycooled anode offgas flow 16. The partially cooled anode offgas flow 16next passes through the air preheater/anode offgas cooler 10, wherein itrejects heat to a fresh cathode air flow 23 and exits as a fully cooledanode offgas flow 17. The fully cooled anode offgas flow 17 enters ananode offgas recycle valve 11, wherein the flow is divided into a firstportion 18 of the anode offgas flow 17 and a second portion 19 of theanode offgas flow 17. While reference is made herein to an anode offgasrecycle valve 11, in some embodiments, the offgas can be divided and/ordiverted using other flow control devices, including, but not limited tobranching junction conduits and the like. The anode offgas recycle valve11 can be any component or combination of components suitable fordividing the anode offgas flow 17. By way of example, the anode offgasrecycle valve 11 may be comprised of a single metering valve, or it maybe comprised of two mass flow controllers.

The first portion 18 of anode offgas enters the anode recyclepressurization device 12 and is elevated in pressure so that it is ableto be combined with a fresh fuel flow 21 to comprise the anode feed 22that is delivered to the HELPER 6. The second portion 19 of anode offgasis combined with an air flow 28 and is oxidized in the ATO 7 to producethe hot ATO exhaust flow 29. The ATO 7 in this embodiment can be a flamecombustor, a catalytic combustor, or any other device with a similarfunction. The hot ATO exhaust 29 passes through the HELPER 6, rejectingheat to the anode feed 22 and exiting as a cooled ATO exhaust flow 30.The amount of air supplied to the ATO 7 is adjusted in order to controlthe temperature of the hot ATO exhaust 29 so as not to exceed themaximum temperature limitation of the HELPER 6. In some embodiments, itmay be preferable to further cool the non-recycled portion 19 of theanode offgas in order to condense out the water vapor from the flowprior to combining the flow with the oxidizing air flow 28.

Turning now to the cathode flow in the embodiment shown in FIG. 1, thefresh cathode air 23 is heated in the air preheater/anode offgas cooler10 by the partially cooled anode offgas flow 16, exiting as a partiallyheated fresh cathode air flow 24. This flow then passes through therecuperative heat exchanger 8, wherein it is heated to a temperaturethat is compatible with the fuel cells and exiting as a fully heatedfresh cathode air flow 25. The fully heated fresh cathode air flow 25 isrouted to the fuel cell cathodes 5, where the cathode reaction reducesmolecular oxygen in the air flow to oxide ions which are thentransported through the electrolyte 3 to the anode 4. The flow providessome convective cooling of the fuel cells 2 and leaves the cathodes as acathode exhaust flow 26 at a temperature exceeding that of the cathodeinlet flow 25. The cathode exhaust flow 26 passes through therecuperative heat exchanger 8, wherein it rejects heat to the partiallyheated fresh cathode air flow 24 and exiting as a cooled exhaust flow27.

The embodiment as described above is able to achieve the statedobjective of providing the capability to adjust the amount of internalreforming through simplified control means by the use of the HELPER.Since the heat source for the HELPER is comprised of the unrecycledanode offgas, the amount of enthalpy available to be recaptured in theHELPER can be adjusted by varying the fuel utilization in the fuelcells, the percentage of anode offgas recycle, or both. In practice, itis important to maintain a sufficient concentration of water vapor inthe anode feed relative to the concentration of oxidizable carbonspecies (a ratio commonly referred to as the “steam-to-carbon” ratio),and this is typically maintained (in an anode recycle system) byrecycling an appropriate percentage of the anode offgas, typically inthe range of 50-60%. Consequently, in some embodiments, it may bepreferable to set the anode recycle percentage to a fixed value and tocontrol the enthalpy made available to the HELPER by varying the fuelutilization alone. Fuel utilization is controlled by adjusting the rateat which the fuel is added to the anode feed, in relation to theelectrical power that is drawn from the fuel cell system.

When fuel utilization is relatively high, the amount of combustiblespecies in the unrecycled anode offgas flow is relatively low. As aresult, the enthalpy made available to the HELPER is also relativelylow, limiting the degree to which the anode feed can be preheated andpartially reformed. This leads to a greater amount of direct internalreforming that must be performed on the anodes. Several advantages arereadily apparent when the system is operating in this mode. First, thethermal efficiency of the system can be maximized by minimizing the rateof fuel addition required for a given electrical output. Second, theamount of stack cooling due to direct internal reforming can bemaximized, thereby reducing the amount of excess cathode air requiredfor stack cooling, which decreases the large parasitic losses due to thecathode air mover.

A further advantage over the prior art, in which a portion of anode feedbypasses an indirect internal reformer, is that all of the anode feedpasses through the HELPER. When the fuel is a natural gas fuel comprisedprimarily of methane with small percentages of higher hydrocarbons, theHELPER can perform the added function of pre-reforming the higherhydrocarbons. Pre-reforming is an adiabatic process wherein water vaporis used to convert higher hydrocarbons to methane, hydrogen, carbonmonoxide and carbon dioxide. By not having any of the anode feedbypassing the reforming catalyst prior to reaching the fuel cell anodes,the risk of anode coking by higher hydrocarbons is greatly reduced.

In some circumstances, it may be preferable to not operate with a highdegree of internal reforming. The embodiment as described provides thecapability of reducing the amount of internal reforming by decreasingthe fuel utilization. When fuel utilization is decreased, the amount ofheat value in the unrecycled anode offgas increases, thereby providingmore enthalpy to the HELPER. This has the effect of increasing thepercentage of fuel that is reformed in the HELPER and decreasing thepercentage that must be internally reformed.

Many suitable construction types for the HELPER can be contemplated,including, but not limited to a bar-plate heat exchanger construction inwhich heat exchange surfaces for the anode feed are coated with areforming catalyst, a plate fin heat exchanger construction in whichheat exchange surfaces for the anode feed are coated with a reformingcatalyst, a concentric annular construction comprised of a metalcylinder, a first convoluted fin structure metallurgically bonded to theinner surface of the cylinder around the entire circumference, a secondconvoluted fin structure metallurgically bonded to the outer surface ofthe cylinder around the entire circumference, and a reforming catalystcoating on one of the first and second convoluted fin structures, andthe like. It should be appreciated that the construction selected forthe HELPER will be highly dependent upon the requirements of eachapplication and the construction selected is not critical to theinvention.

In one example analyzed by the inventor, it was assumed that, the fuelcells were operating at 820° C. with 80% fuel utilization and a cellvoltage of 735 mV, the fuel consisted of methane at 20° C., and that thepercentage of anode offgas recycled was 55%. The analysis of the fuelcell system showed that 92% of the methane fuel would need to bedirectly internally reformed, that the cathode air flow required forproper cooling of the fuel cells would need to be 3.55 times thestoichiometric airflow, and that the calculated thermal efficiency ofthe fuel cells was 63.5% based on the lower heating value of methane.The calculated temperature profiles of the fluids passing through theHELPER are illustrated in FIG. 2. It is evident from the low exhaustoutlet temperature in FIG. 2 that almost all of the enthalpy availablein the exhaust stream has been recovered. It is also evident in FIG. 2that the anode feed has been heated from an inlet temperature ofapproximately 100° C. to an outlet temperature of approximately 510° C.FIG. 3 illustrates the calculated molar fractions of the differentspecies comprising the anode feed flow along the length of the HELPER.FIG. 3 illustrates that substantially no chemical reactions occur overthe first 80% of the HELPER. This effect is due to the temperaturedependency of the reforming reaction kinetics. However, FIG. 3 doesillustrate that the HELPER is capable of heating the anode feed to atemperature where some (approximately 8%) of the methane can bereformed.

The analysis was repeated with the fuel utilization reduced to 65%.Under these operating conditions, the analysis of the fuel cell systemshowed that 76% of the methane fuel would need to be directly internallyreformed, the cathode air flow required for proper cooling of the fuelcells would need to be 3.87 times the stoichiometric airflow, and thatthe calculated thermal efficiency of the fuel cells was 56.8% based onthe lower heating value of methane. It should be appreciated that thecalculated amount of direct internal reforming required of the fuel cellanodes was substantially reduced, with a slight increase in the amountof cathode air required and a slight decrease in the fuel cell thermalefficiency, by decreasing the fuel utilization. The calculatedtemperature profiles of the fluids passing through the HELPER for thisanalysis are illustrated in FIG. 4. FIG. 4 illustrates that the minimumtemperature approach between the two fluids is no longer at the cold endof the HELPER, but is instead located near the center of the length. Asa result, the exhaust stream exits at a higher temperature than in thefirst analysis and the amount of unrecovered enthalpy is somewhathigher. However, the greater enthalpic value of the exhaust streamresults in a higher exit temperature (approximately 590° C.) for theanode feed stream. FIG. 5 illustrates the calculated molar fractions ofthe different species comprising the anode feed flow along the length ofthe HELPER for this analysis. It can be seen that the reformingreactions start significantly sooner than in the previous analysis, atapproximately 50% of the length, and that significantly more of themethane is reformed.

FIG. 6 illustrates an embodiment of the present invention comprising allof the aspects of the embodiment shown in FIG. 1 and further comprisingadditional features that enable both rapid startup of the fuel cellsystem and stable operation at low power output levels, therebysatisfying another object of the invention. Functionally identicalcomponents and flowstreams to those in the embodiment illustrated inFIG. 1 are identified with like reference numerals. Functionally similarbut not identical flowstreams are identified with like referencenumerals, with such numerals in FIG. 6 being followed by a primemodifier.

The embodiment of the fuel cell system 1 illustrated in FIG. 6 includesa startup reformer 31 that can be used to produce a reformate stream 35through partial oxidation of the fuel 21 and an air flow 38. Thereformate stream 35 is comprised of nitrogen, hydrogen, carbon monoxide,carbon dioxide and water vapor. It is well-known as beneficial for thefuel cell anodes to be exposed to a reducing environment during startup.In the embodiment illustrated in FIG. 6, the outlet of the startupreformer 31 is operatively connected to the anode feed inlet of theHELPER 6, so that during startup, the anode feed to the HELPER 22′ iscomprised of the reducing reformate stream 35. In this manner, theanodes can be exposed to a reducing environment during startup. Itshould be appreciated that the primary purpose of the startup reformer31 is to provide the anodes with a reducing environment, andconsequently there is no need for the startup reformer 31 to be of asize suitable for producing reformate at such a rate as is necessary forthe production of any amount of electricity by the fuel cells.

The embodiment illustrated in FIG. 6 further includes a start burner 32,a water pump 34, and a steam generator 33. The start burner 32 producesa hot exhaust flow 41 that combines with the cathode exhaust flow 26. Inthis embodiment, the recuperative heat exchanger 8 receives the combinedstartup burner exhaust 41 and cathode exhaust 26 and transfers heat fromthis flow to the incoming cathode air 24, so that the combined startupburner exhaust 41 and cathode exhaust 26 exit the heat exchanger 8 asthe exhaust flow 27′. The exhaust flow 27′ passes through the steamgenerator 33, wherein it rejects heat to a water flow 36 provided by thewater pump 34, the exhaust flow 27′ exiting the steam generator 33 as acooled exhaust flow 42 and the water flow 36 exiting the steam generator33 as a superheated steam flow 37. The steam flow 37 is connected intothe anode supply piping so that the anode feed 22′ comprises the steamflow 37 when water is supplied by the water pump 34.

FIG. 6 further illustrates a two-position three-way valve 43 with afirst port operatively connected to receive the pressurized anoderecycle flow 20, a second port operatively connected to the anode feedinlet of the HELPER 6, a third port operatively connected to startburner 32, a first position that directs flow from the first port to thesecond port, and a second position that directs flow from the first portto the third port.

The invention contemplates a method of the starting the fuel cell system1 in the embodiment illustrated in FIG. 6, the method comprising thesteps of:

-   -   1. providing an air flow 23;    -   2. providing a reducing reformate flow 35 comprised of a fuel        flow 21 and an air flow 38;    -   3. setting the anode offgas recycle valve 11 so that all of the        anode offgas 17 is diverted to the recycle pressurization device        12 as a recycled anode offgas 18, and flowing the pressurized        recycled anode offgas 20 through the two-position three-way        valve 43, the valve 43 set to a position that directs the        recycled anode offgas 20 to the start burner 32;    -   4. igniting the start burner 32 to combust the recycled anode        offgas 20 using a start burner air flow 39, the exhaust 41 from        the start burner 32 being used to provide a heated cathode air        flow 25 in order to heat the fuel cells 2;    -   5. adding a fuel flow 40 to the start burner 32 in order to        provide a greater flow of exhaust 41;    -   6. setting the anode offgas recycle valve 11 so that some of the        anode offgas 17 is diverted to the ATO 7 as a non-recycled anode        offgas flow 19, providing an ATO air flow 28 to the ATO 7 and        combusting the non-recycled anode offgas flow 19 with the ATO        air flow 28 in the ATO 7 to direct an ATO exhaust flow 29 to the        HELPER 6;    -   7. providing a water flow 36 to the steam generator 33 to        produce a superheated steam flow 37;    -   8. turning off the air flow 38 to the startup reformer 31 so        that the anode feed flow 22′ is comprised of the steam flow 37        and the fuel flow 21, and using the heat from the ATO exhaust 29        to reform a portion of the fuel 21 in the HELPER 6;    -   9. setting the two-position three-way valve 43 to a position        that directs the recycled anode offgas 20 to the HELPER 6;    -   10. producing an electrical current flow from the fuel cells 2;    -   11. turning off the water flow 36; and    -   12. turning off the air flow 39 and the fuel flow 40 to the        start burner 32.

The embodiment as described is also capable of operating at low poweroutput conditions. Since the heat for the reformer is not derived fromthe heat produced by the fuel cell reactions within the stack, but israther derived from the heating value of the anode offgas, it becomespossible to operate the fuel cell system 1 at a low power output and alow fuel utilization, thereby providing a greater amount of enthalpy tothe HELPER 6 by way of the ATO exhaust 29 and externally reforming arelatively large percentage of the anode feed in the HELPER.

The embodiments described above and illustrated in the figures arepresented by way of example only and are not intended as a limitationupon the concepts and principles of the present invention. As such, itwill be appreciated by one having ordinary skill in the art that variouschanges are possible.

1. A fuel cell system comprising: a plurality of solid oxide fuel cellsarranged in a fuel cell stack; an integrated heat exchanger/reformeroperable to partially reform an anode feed prior to entry into the fuelcell stack; an anode tailgas oxidizer; and an offgas flow path extendingaway from an anode side of the fuel cell stack and having a first branchto selectively combine offgas from the anode side of the fuel cell stackwith fuel from a fuel source to comprise the anode feed to the fuel cellstack and a second branch to supply offgas from the anode side of thefuel cell stack to the anode tailgas oxidizer; wherein the integratedheat exchanger/reformer transfers heat from the oxidized offgas from theanode tailgas oxidizer to the anode feed before the anode feed entersthe anode side of the fuel cell stack, and wherein the offgas from theanode tailgas oxidizer provides the sole heat source for the anode feedtraveling through the integrated heat exchanger/reformer.
 2. The fuelcell system of claim 1, wherein a portion of the anode feed not reformedin the integrated heat exchanger/reformer is internally reformed at theanode side of the fuel cell stack.
 3. The fuel cell system of claim 2,wherein relative volumes of the anode feed reformed by the integratedheat exchanger/reformer and internally at the anode side of the fuelcell stack are adjustable.
 4. The fuel cell system of claim 2, whereinan amount of anode feed reformed internally at the anode side of thefuel cell stack is adjusted by varying fuel utilization of the system.5. The fuel cell system of claim 1, further comprising a valvepositioned along the offgas flow path for adjusting relative volumes ofoffgas entering the first branch and the second branch, respectively. 6.The fuel cell system of claim 1, further comprising a compressorpositioned along the offgas flow path for elevating a pressure of theoffgas prior to recycling the offgas through the fuel cell stack.
 7. Thefuel cell system of claim 1, further comprising a compressor positionedalong the first branch of the offgas flow path for elevating a pressureof the offgas prior to directing the offgas through the integrated heatexchanger/reformer.
 8. The fuel cell system of claim 1, furthercomprising a heat exchanger positioned along the offgas flow path topromote heat exchange between the offgas of the anode side of the fuelcell stack and the anode feed.
 9. The fuel cell system of claim 8,wherein the heat exchanger is a recuperator, and wherein a first portionof the heat exchanger is positioned downstream of the anode side of thefuel cell stack and a second portion of the heat exchanger is positioneddownstream of the integrated heat exchanger/reformer.
 10. The fuel cellsystem of claim 1, further comprising a startup reformer fluidlyconnected to the offgas flow path for providing a reformate stream. 11.The fuel cell system of claim 10, wherein the startup reformer isfluidly connected to the first branch of the offgas flow path upstreamof the integrated heat exchanger/reformer.
 12. The fuel cell system ofclaim 10, wherein the anode feed includes at least a portion of thereformate stream.
 13. The fuel cell system of claim 1, furthercomprising a steam generator fluidly connected to the offgas flow pathfor providing a steam flow.
 14. The fuel cell system of claim 13,wherein the steam generator is fluidly connected to the first branch ofthe offgas flow path upstream of the integrated heat exchanger/reformer.15. The fuel cell system of claim 13, wherein the anode feed includes atleast a portion of the steam flow.
 16. The fuel cell system of claim 1,further comprising a second offgas flow path extending away from acathode side of the fuel cell stack.
 17. The fuel cell system of claim16, wherein the offgas flow pass is a first offgas flow path, andfurther comprising a valve positioned along the first offgas flow path,and a startup burner fluidly connected to the second offgas flow path,and wherein the valve selectively directs offgas from the anode side ofthe fuel cell stack to one of the startup burner and the integrated heatexchanger/reformer.
 18. The fuel cell system of claim 17, furthercomprising a heat exchanger positioned along the second offgas flow pathto promote heat exchange between a cathode feed and a combined flow ofan exhaust generated by the startup burner and the offgas from thecathode side of the fuel cell stack.
 19. The fuel cell system of claim16, further comprising a heat exchanger positioned along the secondoffgas flow path to promote heat exchange between offgas from thecathode side of the fuel cell stack and a cathode feed.
 20. The fuelcell system of claim 16, further comprising a steam generator positionedalong the second offgas flow path and fluidly connected to the firstoffgas flow path for providing a steam flow to the anode feed.
 21. Amethod of operating a fuel cell system, the method comprising the actsof: providing a fuel cell stack including a plurality of solid oxidefuel cells; at least partially reforming an anode feed in an integratedheat exchanger/reformer; supplying the at least partially reformed anodefeed to an anode side of the fuel cell stack; removing offgas from theanode side of the fuel cell stack along an offgas flow path; combining afirst portion of offgas from the anode side of the fuel cell stack withthe anode feed to recycle the offgas back to the fuel cell stack;supplying a second portion of offgas from the anode side of the fuelcell stack to an anode tailgas oxidizer; transferring heat from theoxidized offgas from the anode tailgas oxidizer to the anode feed in theintegrated heat exchanger/reformer; and adjusting relative volumes ofthe anode feed reformed by the integrated heat exchanger/reformer anddirectly internally at the anode side of the stack by adjusting the heatcontent of the oxidized offgas.
 22. The method of claim 21, furthercomprising adjusting a rate at which fuel is added to the anode feed tocontrol a fuel utilization of the stack.
 23. The method of claim 21,further comprising adjusting a ratio of the first portion of offgas tothe second portion of the offgas to adjust the heat content of theoxidized offgas.
 24. The method of claim 21, further comprisingadjusting the heat content of the oxidized offgas by varying the fuelutilization of the system.
 25. The method of claim 21, furthercomprising elevating a pressure of at least a portion of the offgas fromthe anode side of the fuel cell stack prior to recycling the offgasthrough the fuel cell stack.
 26. The method of claim 25, furthercomprising elevating a pressure of the first portion of offgas prior todirecting the offgas through the integrated heat exchanger/reformer. 27.The method of claim 21, further comprising providing a heat exchangerpositioned along the offgas flow path transferring heat from the offgasof the anode side of the fuel cell stack to the anode feed downstream ofthe integrated heat exchanger/reformer.
 28. The method of claim 21,further comprising providing a reformate stream wherein the anode feedincludes at least a portion of the reformate stream.
 29. The method ofclaim 28, wherein the act of providing the reformate stream includesproviding the reformate stream to the first portion of offgas upstreamof the integrated heat exchanger/reformer.
 30. The method of claim 21,further comprising providing a steam generator fluidly connected to theoffgas flow path for providing a steam flow.
 31. The method of claim 30,further comprising providing the steam flow to the first portion ofoffgas flow path upstream of the integrated heat exchanger/reformer. 32.The method of claim 21, further comprising removing offgas from acathode side of the fuel cell stack along a second offgas flow path. 33.The method of claim 21, further comprising positioning a valve along aflow of the first portion of offgas, providing a startup burner fluidlyconnected to the second offgas flow path, and selectively directingoffgas with the valve from the anode side of the fuel cell stack to oneof the startup burner and the integrated heat exchanger/reformer. 34.The method of claim 33, further comprising transferring heat from acombined flow of an exhaust generated by the startup burner and theoffgas from the cathode side of the fuel cell stack to a cathode feed.35. The method of claim 32, further comprising transferring heat fromthe second offgas flow path to a cathode feed.
 36. The method of claim32, further comprising transferring heat from the second offgas flowpath to a steam generator fluidly connected to the offgas flow path forproviding a steam flow.
 37. A fuel cell system comprising: a fuel cellstack having an anode side; an integrated reformer/heat exchanger; afirst offgas flow path extending through a first pass of the integratedreformer/heat exchanger; and a second offgas flow path extending througha second pass of the integrated reformer/heat exchanger and in heatexchange relation with offgas flowing through the first offgas flowpath; wherein the second offgas flow path is operatively connected tothe inlet of the anode side of the fuel cell stack downstream of theintegrated reformer/heat exchanger to deliver an anode feed comprisingrecycled anode offgas to the anode side of the fuel cell stack, andwherein an upstream portion of the first offgas flow path is fluidlyconnected to an upstream portion of the second offgas flow path.
 38. Thefuel cell system of claim 37, further comprising an oxidizer positionedupstream of the integrated reformer/heat exchanger along the firstoffgas flow path.
 39. The fuel cell system of claim 37, furthercomprising a valve positioned along the second offgas flow path toselectively direct offgas from the anode side of the fuel cell stack toone of the first pass of the integrated reformer/heat exchanger and astartup burner.
 40. The fuel cell system of claim 39, wherein thestartup burner is operable to receive offgas from the valve and generatean exhaust flow to preheat a cathode feed.
 41. The fuel cell system ofclaim 37, further comprising a third offgas flow path operativelyconnected to the outlet of a cathode side of the fuel cell stack, thethird offgas flow path extending through a third pass of a heatexchanger to preheat a cathode feed.
 42. The fuel cell system of claim41, further comprising a steam generator positioned along the thirdoffgas flow path, the steam generator includes a fourth pass for waterflow and a fifth pass in heat connection with the fourth pass andfluidly connected to the third offgas flow path to allow offgas from thecathode side of the fuel cell stack therethrough.
 43. The fuel cellsystem of claim 42, wherein the steam generator is operable to generatea steam flow and deliver the steam flow to the second offgas flow pathupstream of the integrated reformer/heat exchanger.